1. Field of the Invention
The invention is related generally to the field of interpretation of measurements made by well logging instruments for the purpose of determining the properties of earth formations. More specifically, the invention is related to a method for identification of the extent and direction of fracturing in subsurface formations.
2. Background of the Art
A significant number of hydrocarbon reservoirs comprise fractured rocks wherein the fracture porosity makes up a large portion of the fluid-filled space. In addition, the fractures also contribute significantly to the permeability of the reservoir. Identification of the direction and extent of fracturing is important in reservoir development for two main reasons.
An important reason for identification of fracture direction is that such a knowledge makes it possible to drill deviated or horizontal boreholes with an axis that is preferably normal to the plane of the fractures. In a rock that otherwise has low permeability and porosity, a well drilled in the preferred direction will intersect a large number of fractures and thus have a higher flow rate than a well that is drilled parallel to the fractures. Knowledge of the extent of fracturing also helps in making estimates of the potential recovery rates in a reservoir and in optimizing the production from the reservoir.
Fractures in the subsurface are to a large extent produced by stress fields. Specifically, fracture planes are oriented in a direction orthogonal to a direction of minimum principal stress in the subsurface. The stress field in a fractured formation is anisotropic. A commonly observed effect of an anisotropic stress field or of fracturing is the phenomenon of shear wave birefringence wherein the velocity of shear waves is dependent upon the direction of propagation and the polarization of the shear wave.
The phenomenon shear wave birefringence in subsurface formations was reported by Alford on surface seismic data. Seismic data has a resolution of the order of tens of meters and it is difficult to correlate surface seismic measurements of azimuthal anisotropy with specific reservoir intervals. Subsequent to the work of Alford, there have been other teachings, such as of Winterstein on the use of a “stripping” technique for relating surface measurements of azimuthal anisotropy to subsurface formations. The stripping techniques have a large amount of uncertainty associated with the estimate of the principal directions of shear wave anisotropy.
Becker (U.S. Pat. No. 4,832,148), the contents of which are fully incorporated herein by reference, teaches the use of an acoustic borehole logging method in which traveltimes of shear waves with two different polarizations are measured. By using a coordinate rotation of the measured shear waves, the principal directions may be determined. The principal directions correspond to shear waves having polarization parallel to and perpendicular to the fracture strike, the former having a higher velocity than the latter. This strike direction is often the maximum in-situ stress direction. In U.S. Pat. No. 6,098,021 to Tang et al., the contents of which are fully incorporated herein by reference, radially polarized monopole shear waves are used to determine the extent of anisotropy proximate to the borehole. The birefringence of cross-dipole shear waves that have a lower frequency than the monopole waves are then used as an indication of shear wave anisotropy further away from the wellbore in the formation.
The method taught by Tang is thus an improvement over Becker insofar as it is possible to distinguish, in a qualitative manner, between near-borehole effects and effects further away from the borehole. A drawback of shear wave birefringence measurements is their inability to distinguish between anisotropy caused by stress and anisotropy caused by fracturing. Though the fracturing may be caused by stress anisotropy, being able to delineate fractures is important in well planning.
In addition to their effects on elastic wave propagation, electrical anisotropy is also present in the subsurface. It should be noted that in the present application, the terms “elastic” and “acoustic” are used interchangeably, although the latter term is not technically correct. In U.S. Pat. No. 4,924,187 to Sprunt et al, the contents of which are fully incorporated herein by reference, a core sample from a subterranean formation is shaped to provide a plurality of parallel, planar outer surfaces. Electrical resistivity is measured in each of the azimuthal directions through the core sample which are perpendicular to each of the pairs of parallel, planar outer surfaces for each of a plurality of differing fluid saturations within the core sample. A logarithmic plot is made of measured resistivity versus water saturation for each of the azimuthal directions through the core sample for which resistivity was measured. If the same logarithmic plot is obtained for all measured azimuthal directions, the core sample is identified as being electrically isotropic. If different logarithmic plots are obtained for at least 2 azimuthal directions the core sample is identified as being electrically anisotropic.
There are two conclusions that may be drawn from the results shown by Sprunt shown in FIG. 1. The first is that there is relatively little azimuthal variation in electrical anisotropy on a core sample compared to differences between measurements made parallel to and orthogonal to the bedding plane. The differences are particularly large at low water saturation. The azimuthal variations are of the order of a few percent whereas the resistivity perpendicular to the bedding plane may be five times the resistivity parallel to the bedding plane. The second conclusion is that the measured anisotropy is dependent upon the water saturation. The latter effect suggests that measurements of azimuthal variations in electrical anisotropy may be indicative of fracturing and/or hydrocarbon saturation.
U.S. Pat. No. 6,191,586 to Bittar teaches an apparatus and method for implementing azimuthal capabilities on electromagnetic wave resistivity well logging tools. The apparatus comprises a structurally simple antenna shield positioned around either the transmitting or receiving antennas, or both, positioned on the well logging tool on the drill string. The shields partially surround the tool and provide an electromagnetic barrier for either the transmission or reception of electromagnetic waves, as the case may be. Positioned on the shield are appropriately placed and sized windows through which electromagnetic waves may either be transmitted or received, depending upon the function of the antenna that the shield surrounds. One of the teachings of Bittar is the use of the device for estimating the dip of the formation (viz., inclination of the tool axis to the normal to the bedding plane). The effects of dip can be quite large given the differences noted in FIG. 1 between the vertical and horizontal resistivities. The tool of Bittar uses transmitter antennas with the coil axis parallel to the tool and the borehole and is not designed to detect the smaller effects due to any oriented fracturing in the formation.
Heavy muds may induce fractures when the mud pressure exceeds the rock strength. In hydrocarbon basins, the maximum stress is often in the vertical axis and the minimum stress is in the horizontal axis. For this reason the fracture will tend to be vertical and follow the direction towards maximum stress levels. From the theory of mechanics, it can be shown that the normal to the fracture direction is a direction of minimum principal stresses, so that the preferential fracture strike direction is aligned with the maximum in-situ stress. Previous investigations have suggested that, except in very shallow wells, most hydraulically created fractures will be vertical or nearly vertical. The fractures change the formation's mechanical and electrical properties and thus influence both acoustic and induction resistivity logs. It is important to be able to determine from the depth of fracturing whether they are induced by the drilling process or whether they are preexisting fractures. The latter are important from the standpoint of reservoir development whereas the former may only be indicative of the stress field in the proximity of the wellbore.
There is a need for a method of determination of the extent and direction of fracturing in subsurface earth formations. Such a method should preferably be fast in operation in the sense that it should be possible to acquire data at normal logging speeds. The present invention satisfies this need.